a few ideas...

I think a very effective way to get the message across is to use humor
as well as poignancy.

1)Long day?…

Pour

yourself a strong THINK
@ ThinkAtheist.com

2)Feel like faith is an ancient fossil?

Discover the missing THINK

@ ThinkAtheist.com

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This is deceptively simple but I really think that the simplicity of the comment may stick with a driver and cause he/she,
if only for a second, to imagine a world without hell. A small nugget of
desire for disbelief. Facts, as we know, don't always help. Spur them
to want to see the other side.

I like the "There is no HELL" one. Christians get defensive about heaven but come on... wouldn't everyone like to believe that no one goes to Hell. Of course, religious people would still be pissed over it. Love all the ideas.